Traditionally, continuous casting of light metal ingot has followed the practice of introducing molten metal into one end of an open-ended mold and withdrawing a solid or partially solidified ingot from the opposite end. Typically, the casting mold is relatively short in the axial direction and is hollow or otherwise adapted to receive a liquid cooling medium, such as water, which chills and solidifies the ingot meniscus. The water is then discharged from the mold and continues to chill the ingot as it contacts the ingot surface. Molds are preferably constructed of aluminum but may also be copper, bronze or another material which exhibits high thermal conductivity.
U.S. Pat. No. 4,166,495 issued to Yu discloses an ingot casting method for controlling the withdrawal of heat from the surface of a cooling ingot including mixing a gas, such as CO.sub.2, with the liquid coolant, typically water, before the liquid coolant is applied to the ingot surface. When the gas containing liquid coolant is applied to the mold during the initial stages of casting, the gas mixed in the liquid coolant acts to retard the rate of heat extraction of the liquid coolant. When the amount of gas mixed with the liquid coolant is reduced, the rate of heat extraction by the mold is increased. The increased rate of heat extraction is used on subsequent portions of the emerging ingot length.
The method of U.S. Pat. No. 4,166,495 is a commercially successful method of retarding the cooling effect of the liquid coolant and has come to be known in the aluminum industry as the Alcoa 729 process. A preferred coolant for the process is water and one preferred gas is CO.sub.2. Other gases which are substantially insoluble in water, such as for example air, may also be used in practicing the method of U.S. Pat. No. 4,166,495.
U.S. Pat. No. 4,693,298 issued to Wagstaff discloses a means an technique for casting metals at a controlled direct cooling rate. The method of U.S. Pat. No. 4,693,298 involves mixing liquid coolant and a gas which is substantially insoluble in the liquid coolant by discharging the gas through jets. The jets release the gas in the flowing liquid coolant as a mass of bubbles that tend to remain discrete and undissolved in the coolant as the coolant on the surface of the ingot.
Although the Alcoa 729 process is economical and effective, it is improvable. The amount of gas mixed with the liquid coolant for the best results in the process can vary with changes in temperature, mixing pressure, and water quality and adjustments are appropriate for the best results. The ability of the gas to retard the heat of extraction of the liquid coolant is determined by the volatility of the liquid, which depends on the concentration of gas mixed in the liquid coolant, the temperature of the liquid coolant, the velocity of coolant flow and the coolant quality of the liquid coolant. The term "quality" as used herein means the chemistry of the liquid coolant and it includes properties such as pH, alkalinity, dissolved and suspended solids, surface tension and ionic species.
In copending U.S. Ser. No. 366,759, filed Jun. 14, 1989, now U.S. Pat. No. 4,987,950 applicant has disclosed a method for continuously monitoring the cooling capacity of a coolant containing bubbles. In one embodiment, the method comprises the steps of: (a) detecting the number density of bubbles within a predetermined size range and (b) comparing the number density to a predetermined number and if necessary varying the amount of gas that is being mixed with the liquid coolant so that the number density obtained is within said predetermined range. In a preferred embodiment, a laser is used to detect the relative number density of the bubbles in water that fall within a predetermined size range. The detection is accomplished by focusing the laser on a device which detects the scattering of laser light by the bubbles.
The method of copending U.S. Ser. No. 366,759 is quite useful in monitoring and controlling the heat capacity of the liquid medium in commercial plants. The method has been found to work at a desirable level even though the coolant quality and temperature may vary. However, it has been found that long term instrument reliability is affected by fouling of the of the light sampling system due to a build-up of slime, dirt, corrosion products and other dissolved and suspended debris on windows which are used to isolate the laser and sensing device from the water. The resulting accumulation of material deleteriously affects the sensitivity of the sensing device and can lead to inaccurate measurements. Cleaning is accomplished by partially dismantling the bubble detector. This represents a significant cost in maintenance and down time.
Accordingly, it would be advantageous to provide an economical and effective method of monitoring and controlling the cooling effect of the liquid medium at a desirable level that does not require disruptive maintenance.
The primary object of the present invention is to provide a method and apparatus for removing deposits on an optical cell used for monitoring the cooling effect of a first liquid containing bubbles or particles of a second liquid.
Another object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which permits the in-situ cleaning of window used in the process of monitoring the rate of heat extraction from a liquid coolant in which gas has been mixed to regulate the cooling rate of the coolant when it is used to cool the surface of a continuously cast ingot.
Still another object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which will apply ultrasonic energy to windows used in monitoring and/or controlling the cooling capacity of a liquid coolant containing gas bubbles.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.